The presence of sulfur in lime products produced from the calcining of limestone is a persistent problem where metallurgical lime is desired. Lime used in metallurgical processes, such as steel making, as a fluxing material is required to have a sulfur content as low as possible. Such sulfur can be present either in the limestone which is calcined, or in the fuel that is used to produce the hot combustion gases for calcination, with such sulfur generally present as sulfur dioxide in the hot gases. Steel mills typically require lime that is very low in sulfur, usually lower than 0.1 percent and often as low as 0.03 percent.
When sulfur present in the coal combusted to form the hot gases for calcination, usually as sulfur dioxide, reacts with calcining lime, calcium sulfate is formed as follows: EQU caO+SO.sub.2 +1/2O.sub.2 .fwdarw.CaSO.sub.4
Such a reaction occurs during combustion when excess air is used. Thus, some of the sulfur originally present in the carbonaceous fuel ends up as part of the lime product produced, while some also is carried through the calcining system and ends up in the exhaust gas cleaning apparatus of a calcining kiln system, typically a bag house or an electrostatic precipitator.
In addition, as above described, the limestone charge to a rotary kiln may contain sulfur compounds, primarily as pyrites, which upon calcination of the stone will produce sulfur dioxide, with oxygen present, generally according to the formula: EQU 51/2O.sub.2 +2FeS.sub.2 +heat.fwdarw.Fe.sub.2 O.sub.3 +4SO.sub.2
which requires the need to keep any sulfur affects of the carbonaceous feed at a minimum.
Where the rotary lime kiln system contains a preheater for the limestone charged, using direct contact of the limestone with hot combustion gases exiting the kiln, sulfur dioxide in the combustion gases can combine with the surface areas of the limestone charge, especially the particles that are calcined on the surface and are most reactive.
Because of the disastrous affect of sulfur in lime used in steel fluxes, where coal is used as a fuel in a calcining system a very low sulfur content coal, an expensive commodity, is used. In addition to the added cost involved using very low sulfur content coal, even slight variations in coal chemistry can cause formation of a less desirable lime product.
It is an object of the present invention to provide a process for calcination of limestone to lime to produce a very low sulfur content lime product while using sulfur-containing fuels, such as sulfur-containing coal, for combustion in a rotary lime kiln to provide heat required for the calcination.